This invention relates generally to an optical system with means for aberration suppression and more particularly to an aberration error correction optical system for use in connection with optical information processing apparatus, such as, optical disk recorders and players, optical digital recording apparatus, laser printers, magnetooptic recording and reproducing systems, optical storage information systems and the like.
In one type of conventional autofocusing/autotracking optical systems for use in connection with an optical head in optical information processing apparatus, detection of displacement of the focused light spot relative to the surface of an optical recording medium is accomplished by mechanically moving the objective lens in the system via a focusing lens drive means such as an actuator, e.g., an electromagnetic coil or the like, in accordance with the amount of detected displacement. An example of such a conventional system is disclosed in FIG. 1 relative to optical head 10. Head 10 comprises a stationary section 12 and a movable section 14 with stationary section 12 fixed to the chassis of an optical information processing apparatus. Stationary section 12 contains semiconductor laser 16, objective lens 18 and beam splitter 20. The divergent output beam from laser 16 is collimated by objective lens 18 onto beam splitter 20 and thereafter the beam is directed to movable section 14.
The movable section 14 contains a mirror 26, objective lens 28 and means 31 to move lens 28 in focusing and transverse track directions, F and T, so that image spot 30 is maintained focused on the surface of recording medium 32. Thus, the beam output of beam splitter 20 is directed onto mirror 26 which folds the beam through objective lens 28 focusing the beam to an image spot 30 on the surface of recording medium 32. Means 31 may be, for example, an actuator that moves lens 28 in the focusing direction, F, along the optical axis or path of head 10. Actuator 31 also moves lens 28 in the transverse track direction, T, to recenter the beam on a designated track on recording medium 32. Such tracking may also be accomplished by a galvano-mirror as is known in the art.
During the random accessing of data on recording medium 32, movable section 14 may be translated by means, such as voice coil motor (VCM) or the like (not shown), which translation is indicated by arrow 34 in FIG. 1.
Light reflected from recording medium 32 at image spot 30 is returned to beam splitter 20 via mirror 26 and is directed onto photodetector 24, e.g., a photodiode, via objective lens 22. Lens 22 produces a pattern on photodetector 24 which varies according to focus and tracking of image spot 30. The signal developed at photodetector 24 is employed in a focal displacement detection circuit (not shown) to produce focusing error and tracking error signals corresponding to the direction and amount of displacement and deviation of the image spot respectively in the focusing direction, F, and the transverse track direction, T. A focusing error signal contains information about the magnitude and direction of a displacement between the focal plane of lens 28 and the surface of recording medium 32 in the direction of the optical axis of system 10. A tracking error signal contains information about the magnitude and direction of a lateral deviation between a focused spot 30 and a designated track of recording medium 32. Thus, objective lens 28 may be moved in focusing direction, F, and in the transverse track direction, T, via actuator 31 based upon information received at photodetector 24, so that the position of image spot 30 will be monitored and maintained on track and in focus relative to recording medium 32. Examples of such types of focusing systems are found in U.S. Pat. Nos. 4,654,839 and 4,725,721.
An important factor in designing an autofocusing/autotracking system is the consideration of optimization of the optical head access time in performing read/write functions relative to an optical information processing apparatus, such as an optical disk memory. Thus, the miniaturization of the optical head approach has been proposed as a way to reduce the weight and mass of head movable section 14 and, therefore, reducing inertia forces, in order to attain higher head random access speeds. See, as an example, U.S. Pat. No. 4,734,905.
However, with such an optical head, there remains the problem of correction for focusing error caused by changes in the height between the optical head focal plane and the surface of the recording medium 32 as well as variations in recording medium thickness. Further, there are limits to the improvement of access time cannot be improved due to the weight of the optical head movable section 14, in particular, the weight and mass of actuator 31 which generally weighs between 5 to 10 grams. Thus, in order to significantly shorten the access time of head 10, the weight associated with movable section 14 would preferably be reduced by removal of actuator 31 from movable section 14 and the placement of the focusing function in stationary section 12. For example, a movable objective lens with actuator 31 may be provide in stationary section 12 rather than in movable section 14. However, aberrations are introduced with the movement of such a lens so that refocusing is not accurately accomplished due to the inability to maintain symmetric light ray correspondence between light source 16 and imaged spot 30 upon movement of the objective lens.
In another approach, objective lens 28 may be maintained stationary and image spot 30 at recording medium 32 may be moved or repositioned by moving light source 16 in stationary section 12 in the direction of the optical axis. However, in moving source 16, aberrations occur in both objective lens 18 and 28 so that, again, refocusing is not accurately accomplished due the inability to maintain symmetric light ray correspondence between light source 16 and imaged spot 30 upon movement of light source 16.
Thus, it is an object of this invention to provide for focusing of image spot 30 by movement of light source 16 or by movement of an objective lens in optical system 10 with the suppression of aberration that is generated due to such movement. It is another object of the present invention to provide means for maintenance of a good beam on a focal point in an optical system while suppressing aberration due to movement of optical components in the optical system to achieve such maintenance. It is another object of this invention to provide an optical head having a stationary section and a movable section that is capable of achieving high speed random accession in optical information processing apparatus beyond that achieved in the present state of the art due to the significant reduction of the mass necessary in the movable section of the optical head while maintaining a focused light beam substantially free of aberration.